The present invention relates generally to wireless communications. More particularly, the present invention relates to low noise amplifiers (LNAs) for use in wireless communication devices.
In wireless communication systems, a plurality of wireless communication devices (WCDs) communicate with one or more base stations within areas known as cells. A wireless communication system can include a variety of types of WCDs, including, for example, wireless telephones and devices having wireless communication capabilities, such as personal digital assistants (PDAs) and modems for use with laptop computers. Wireless communication functions can also be incorporated into other types of devices, such as automobiles. With wireless capabilities incorporated into an automobile design, a driver can obtain real-time, location-based traffic, weather, and navigation information, as well as roadside assistance and vehicle condition alerts.
Within each cell, several WCDs may communicate with a base station simultaneously using a single frequency band. Sharing of the frequency band can be accomplished using any of a variety of multiple access techniques. For example, some wireless communication systems use time division multiple access (TDMA) or Global System for Mobile (GSM) technologies in which WCDs communicate during allocated time slots. Some other wireless communication systems use other multiple access technologies, including, for example, frequency division multiple access (FDMA), amplitude companded single sideband (ACSSB) and other amplitude modulation (AM) schemes.
One technology that has enjoyed rapid growth is code division multiple access (CDMA). In CDMA systems, speech or data is converted to a digital form, which is then transmitted as a radio signal. Each call is distinguished by a unique code. In particular, each WCD uses a unique spreading code to modulate the signals it transmits and to demodulate the signals it receives. This code is added to the information data, e.g., the voice data, and modulated onto the carrier. An identical code is used in the receiver that is used to correlate the code with the carrier. The correlation process passes only data that matches the code. Thus, non-valid signals, e.g., signals from other users, are not decoded and appear as noise. As a result, minimal interference between WCDs is achieved. Accordingly, several WCDs can share a single frequency band. Further information regarding CDMA systems is set forth in the well-known IS-95 standard.
Many WCDs, such as wireless telephones, are battery-powered. For such WCDs, power consumption is an important design consideration. That is, it is desirable to reduce power consumption to the extent possible consistent with other design considerations. For example, the operating environment often dictates high performance for the RF front-end, particularly for CDMA systems, which receive and transmit simultaneously. High performance for the RF front-end is important because the presence of an interference signal as the transmit channel approaches its maximum power output generally causes cross-modulation of the transmit signal envelope. This results in xe2x80x9cin-bandxe2x80x9d interference in the receive channel. Such in-band interference can result in degraded received signal quality, potentially resulting in dropped calls.
To maintain a tolerable level of cross-modulation, an IIP3 characteristic of an LNA or mixer channel can be adjusted to a high value. The IIP3 characteristic measures the signal strength at which the power of the third-order distortion energy of a gain stage is as strong as the fundamental signal energy. It can be shown that, simultaneously with a low noise figure, the low noise amplifier (LNA) should also have a very high IIP3 characteristic. See, e.g., V. Aparin, B. Butler, P. Draxler, xe2x80x9cCross Modulation Distortion in CDMA Receivers,xe2x80x9d IEEE International Microwave Symposium, Boston, June 2000. Using a high IIP3 value increases the linearity of an LNA or mixer channel, but also increases the bias current to the LNA or mixer channel. In a bipolar transistor LNA design, the IIP3 characteristic typically increases with increasing current consumption. However, the noise figure also increases at high currents. As a result, in some conventional implementations, a high IIP3 characteristic often results in a poor noise figure and excessive current consumption. High current consumption results in a high drain on battery power, thus reducing both talk and standby time for a wireless telephone.
Several conventional techniques have been offered to obtain a higher IIP3 characteristic without sacrificing the noise figure or current consumption. For example, at RF frequencies, the IIP3 characteristic is strongly affected by the presence of low-frequency distortion products. See, e.g., V. Aparin, C. Persico, xe2x80x9cEffect of Out-of-Band Termination on Intermodulation Distortion in Common-Emitter Circuits,xe2x80x9d IEEE MTT-S Dig., vol. 3, June 1999, pp. 977-980. In a two-tone test, nonlinearities will cause the generation of several mixing products. When two signals with different frequencies f1 and f2 are applied to a nonlinear system, the output will typically exhibit some components that are not harmonics of the input frequencies f1 and f2. One such component is attributable to a phenomenon known as intermodulation (IM).
Intermodulation results from mixing or multiplication of the two signals when their sum is raised to a power greater than unity. For example, one intermodulation mixing product occurs at the frequency (f1xe2x88x92f2). This can be viewed as a low-frequency modulation of the operating point. Due to unavoidable internal feedback inside the transistor as well as external feedback, the (f1xe2x88x92f2) product will mix again with f1 and f2, thus creating in-band distortion products at the IM3 (third-order intermodulation) frequencies (f1xe2x88x92f2)+f1=2*f1xe2x88x92f2 and (f2xe2x88x92f1)+f2=2*f2xe2x88x92f1. These IM3 products are a particularly significant source of distortion in RF systems. If a weak signal accompanied by two strong interference sources experiences third-order nonlinearity, then one of the intermodulation products falls in the band of interest, corrupting the desired component. In order to obtain a high IIP3 characteristic, it is beneficial to ensure that a low impedance is presented to these IM3 products, essentially shorting them out.
According to various embodiments, a feedforward nonlinearity cancellation scheme is used to improve the linearity of a low noise amplifier (LNA). An LNA incorporates a main amplifier and an auxiliary amplifier. The auxiliary amplifier may be implemented as a very low power auxiliary amplifier having a very low linearity. The auxiliary amplifier is coupled to receive the same input signal as the main amplifier. The outputs of the main amplifier and the auxiliary amplifier are also coupled. The output of the auxiliary amplifier contains third-order intermodulation (IM3) products that are of similar amplitude, but opposite phase, to the IM3 products generated by the main amplifier. With the outputs of the main amplifier and the auxiliary amplifier coupled, their respective IM3 products are summed together and effectively cancel each other out. As a result, the output of the LNA contains substantially no IM3 products, and the linearity of the LNA is substantially improved.
In one embodiment, a low noise amplifier includes first and second amplifiers coupled to receive an input RF signal. The first amplifier generates a first output signal as a function of the RF input signal. The first output signal has a first intermodulation component. The second amplifier, which is coupled to the first amplifier, generates a second output signal as a function of the RF input signal. The second output signal has a second intermodulation component that has substantially similar amplitude and substantially opposite phase to the first intermodulation component. The first and second amplifiers are arranged, e.g., AC coupled, to generate a third output signal as a function of a sum of the first and second output signals. The first and second intermodulation components substantially cancel each other.
In another embodiment, an integrated circuit includes first and second amplifiers coupled to receive an input RF signal. The first amplifier generates a first output signal as a function of the RF input signal. The first output signal has a first intermodulation component. The second amplifier, which is coupled to the first amplifier, generates a second output signal as a function of the RF input signal. The second output signal has a second intermodulation component that has substantially similar amplitude and substantially opposite phase to the first intermodulation component. The first and second amplifiers are arranged, e.g., AC coupled, to generate a third output signal as a function of a sum of the first and second output signals. The first and second intermodulation components substantially cancel each other.
In still another embodiment, a wireless communication device (WCD) includes an antenna arranged to receive an RF signal. A first amplifier is operatively coupled to the antenna and generates a first output signal as a function of the RF signal. The first output signal has a first intermodulation component. A second amplifier is coupled to the first amplifier and generates a second output signal as a function of the RF signal. The second output signal has a second intermodulation component of substantially similar amplitude and substantially opposite phase to the first intermodulation component. The first and second amplifiers are arranged, e.g., AC coupled, to generate a third output signal as a function of a sum of the first and second output signals. The first and second intermodulation components substantially canceling each other. A downconversion arrangement is coupled to the first and second amplifiers and generates a baseband signal as a function of the third output signal. A demodulator arrangement is coupled to the downconversion arrangement and demodulates the baseband signal.
In yet another embodiment, a method to generate an amplified signal includes receiving an RF signal. A first output signal is generated as a function of the RF signal. The first output signal has a first intermodulation component. A second second output signal is generated as a function of the RF signal. The second output signal has a second intermodulation component of substantially similar amplitude and substantially opposite phase to the first intermodulation component The amplified signal is generated as a function of a sum of the first and second output signals. The first and second intermodulation components substantially cancel each other. This method may be embodied in a low noise amplifier.